The thin film magnetic recording disk in a conventional drive assembly typically consists of a substrate, an underlayer consisting of a thin film of chromium (Cr) or a Cr alloy, a cobalt-based ferromagnetic alloy deposited on the underlayer, and a protective overcoat over the magnetic layer. The word "magnetic" will be used to mean ferromagnetic, antiferromagnetic, ferrimagnetic or any other magnetic material suitable for magnetic recording. A variety of disk substrates such as NiP-coated AlMg, glass, glass ceramic, glassy carbon, etc., have been used. The microstructural parameters of the magnetic layer, i.e., crystallographic preferred orientation (PO), grain size, anisotropy and magnetic exchange decoupling between the grains, play key roles in the recording characteristics of the disk. The Cr underlayer is mainly used to control such microstructural parameters such as the PO, the unit cell size and grain size of the cobalt-based magnetic alloy.
One variation of the layer structure described above uses a very thin initial seed layer on the substrate to establish an appropriate nucleation base for the underlayer. Various materials have been used or proposed for seed layers, for example, Al, Cr, Ni.sub.3 P, Ta, C, W, FeAl and NiAl. Laughlin, et al., have described use of an NiAl seed layer followed by a Cr underlayer and a CoCrPt magnetic layer. The NiAl seed layer with the Cr underlayer was said to induce the [1010] texture in the magnetic layer. (See "The Control and Characterization of the Crystallographic Texture of Longitudinal Thin Film Recording Media", IEEE Trans. Magnetic. 32(5) September 1996, p. 3632).
The PO of the various materials forming the layers on the disk, as discussed herein, is not necessarily an exclusive orientation which may be found in the material, but is merely the most prominent orientation. When the Cr underlayer is sputter deposited at a sufficiently elevated temperature on a NiP-coated AlMg substrate a [200] PO is usually formed. This PO promotes the epitaxial growth of [1120] PO of the hexagonal close-packed (hcp) cobalt (Co) alloy, and thereby improves the magnetic performance of the disk. The [1120] PO refers to a film of --hexagonal structure whose (1120) planes are predominantly parallel to the surface of the film. (Likewise the [1010] PO refers to a film of hexagonal structure whose (1010) planes are predominantly parallel to the surface of the film).
In the prior art the optimal underlayer structure was believed to be one with as little deviation from the target PO as possible. For example, if [200] PO was the design goal for the underlayer, then it was thought that the more narrow the distribution of the orientation of the grains, the better and ideally every grain would be [200].
Alloys of chromium have been used for the underlayer. For example, CrTi and CrV have been used. The addition of limited amounts of titanium or vanadium modifies the lattice parameters by atomic substitution, but the crystalline nature of the underlayer is not modified.